Two-stage LED driver with buck PFC and improved THD

ABSTRACT

A two-stage light emitting diode (LED) driver for powering an LED load at a substantially constant current, and related methods and systems. The first or front end stage of the LED driver includes a buck topology power factor correction (PFC) circuit, the buck PFC circuit and a PFC controller. The second stage of the LED driver includes a conventional isolation and regulator circuit configured to receive the DC voltage and DC current output by the buck PFC and then to provide the substantially constant current to the LED load. By multiplying the rectified input voltage sensed by the PFC controller, the input AC current drawn by the buck PFC circuit has a much improved total harmonic distortion (THD), which is achievable without the need for using an expensive PFC controller. The rectified input voltage is multiplied using a Zener diode ladder.

I. FIELD OF THE INVENTION

The present invention relates generally to power supplies and, moreparticularly, to a high-efficiency, two-stage light emitting diode (LED)driver having a front end buck topology power factor correction (PFC)circuit, running in transition mode, that achieves improved totalharmonic distortion (THD) results without the need for a high-cost PFCcontroller.

II. BACKGROUND OF THE INVENTION

Power converters are used in a variety of portable electronic devices,including laptops, mobile devices, cellular phones, electronic digitalpads, video cameras, digitals cameras, and the like. In addition, powerconverters may be used in non-portable applications, such asliquid-crystal display (LCD) backlighting, automotive lighting, andother general purpose or specialty lighting.

Power converters come in many forms. Some converters are DC-DCconverters, which convert a Direct Current (DC) input voltage to adifferent DC output voltage. AC-AC converters convert one AlternatingCurrent (AC) input voltage to a different AC output voltage. DC-ACconverters convert a DC input voltage to an AC output voltage, and AC-DCconverters convert an AC input voltage to a DC output voltage.

Conventional AC-DC power converters typically include a diode bridgerectifier stage (i.e., a bridge or full-wave rectifier) and a bulkstorage capacitor. The incoming AC voltage is generally provided by anAC power supply or AC line, which is converted to a DC output voltagewhen run through the diode bridge rectifier and bulk storage capacitor.This DC voltage is typically further processed by a converter, whichgenerates an output signal that is applied across a load.

In this configuration, the rectifying circuit only draws power from theAC line when the instantaneous AC voltage is greater than the voltageacross the bulk storage capacitor, resulting in a non-sinusoidal currentsignal that has high harmonic frequencies. A drawback with thisconfiguration is that the power factor or ratio of real power toapparent power is usually very low. Thus, the converter draws excesscurrent but fails to use the excess current to perform or accomplish anycircuit functions.

To address the power factor issue, it is common to couple a power factorcorrection (PFC) stage to the diode bridge rectifier, which improves theuse of current drawn from the main AC line by shaping it to be moresinusoidal. Generally, power converters that include PFC stages areeither double-stage or single-stage power converters.

A converter having a double-stage PFC architecture allows foroptimization of each individual power stage. However, this type oftwo-stage architecture uses many components and processes the powertwice.

A converter having a single-stage PFC architecture uses fewer componentsand processes the power one time, which can improve efficiency and canbe more reliable than a double-stage PFC architecture. But, a majordrawback with the single-stage architecture is that it has a largeoutput current ripple, which is at twice the AC line frequency. Themagnitude of this ripple can overdrive conventional feedbacknetworks—forcing them outside of their linear response region ordegrading their ability to maintain a high power factor.

One technique for smoothing out or decreasing the large output currentripple is to couple a filtering capacitor, having a large capacitancevalue, to the output filter network. However, although a filteringcapacitor having a large capacitance value smoothes out the large outputcurrent ripple delivered to the load without interfering with thecontrol loop, such a filtering capacitor is usually an electrolyticcapacitor that tends to be large and expensive and tends to degradecircuit reliability.

In addition, the large capacitance of such a filtering capacitor slowsthe response time of the control loop—resulting in excessive current,which can overdrive, and potentially damage, the load. The excessivecurrents typically occur when the load is connected to a pre-poweredconverter (e.g., “hot plug”, “hot insertion”). The output capacitor atthis point is fully charge to the maximal output voltage; thus, theenergy stored in it can damage the load right at the connection of it tothe converter.

As a solid state light source, LEDs are being used more and morefrequently due to their superior longevity, low-maintenancerequirements, and continuously-improving luminance. In low-powerlighting applications, the cost of LED drivers that are used to powerLED loads is a critical design consideration. Such costs, however, mustalso be weighed against the necessary performance criteria of LEDdrivers, which must not only be efficient but also generate minimalripples in the output current provided to the LED load. Large currentripples reduce the reliability, longevity, and luminance output of theLEDs, which is obviously not desirable.

Although there are numerous LED driver designs that use either two-stagepower converters or single-stage power converters, one common type ofLED driver is a two-stage PFC converter that includes an active PFCstage followed by a DC-DC converter stage. The active PFC stage providesa near unity power factor and a low total harmonic distortion (THD)across the entire universal input voltage range, while the DC-DC stageis used to provide tight regulation and control on the current outputprovided to the LED load. The DC-DC converter stage may also be referredto as a downstream isolation and regulator circuit, since it isconfigured to receive the DC voltage and the DC current output by theactive PFC stage and then to provide a substantially constant current tothe load (or LED load).

The active PFC stage is typically accomplished with a boost powertopology. A drawback of such conventional designs, however, is the factthat these two stages require two independently controlled powerswitches and two control circuits (or “controllers”). The two-stagedesign suffers from an increased component count and ahigher-than-desired cost.

Although it would be cheaper to employ a passive PFC as the first stage,such topology architectures usually cannot provide the necessaryefficiency required by energy regulations or minimal current ripplesrequired by the LED load. Another drawback to such two stage LED driverdesigns is that each LED driver is typically configured for one specificoutput current level. For each application requiring a different outputcurrent, a different LED driver is typically necessary.

For these reasons, there is a need for an LED driver that uses an activePFC stage, while still achieving necessary efficiency and minimizingoutput current ripples required by the LED load.

There is a need in the industry to be able to use a buck (step-down)topology, as the active PFC stage, that functions in transition mode(also referred to as boundary conduction mode or critical conductionmode operation), using one of the many low cost control chips, orintegrated circuits (ICs), that are typically only designed to be usedwith either flyback or boost topologies. However, such low cost ICsgenerally do not work well with buck topologies because they do notprovide good THD results (i.e., they often draw more than desired—orrequired by law or regulation—power from the AC power supply).

Although there are some active and expensive control chips or ICs thathave been developed specifically for use with buck topologies to improvetheir THD results, there remains a need in the industry for enablingbuck topologies to be used with the above-mentioned, lower-cost controlchips or ICs, while still being able to achieve good THD results.Further, there is a need in the industry for a single LED driver thatcan provide at least two different output currents, preferablyswitchable by the user, so that such single LED driver can be used witha wider range of LED load applications.

III. SUMMARY OF EMBODIMENTS OF THE INVENTION

Given the aforementioned deficiencies, a need exists for systems,methods, and devices providing a low cost and efficient LED driver.Particularly, what are needed are systems, methods, and devices thatenable an active buck topology, functioning in transition mode, to beused as a first PFC stage of an LED driver whereby the buck topology isdesigned in such a manner that it can be controlled with a low costcontrol chip that is typically only used with boost or flybacktopologies, while still achieving good THD results. Further, what areneeded are systems, methods, and devices that enable a flyback currentcircuit to be used as a second stage of an LED driver, whereby theflyback circuit includes a switch or jumper setting selectable by theuser that enables the LED driver to be toggled or switched between twodifferent output currents—depending upon the requirements of the LEDload being powered by the LED driver.

Embodiments of the present invention provide a light emitting diode(LED) driver for powering an LED load at a constant or substantiallyconstant current. The LED driver includes a buck topology power factorcorrection (PFC) circuit having a low cost PFC controller, the buck PFCcircuit configured to draw an AC input current having an original totalharmonic distortion (THD) and further configured to sense a rectifiedinput voltage from a full-wave rectifier. The LED driver also includes alow voltage flyback circuit configured to receive the DC voltage and DCcurrent output by the buck PFC circuit. A passive voltage multipliercircuit is configured to multiply the sensed rectified input voltageprovided to the PFC controller by a value on N, where N is a wholenumber greater than 1, which causes the AC input current drawn by thebuck PFC to circuit to have an improved second, lower THD better thanthe original THD.

In some embodiments, an improved second stage of an LED driver includesa low voltage flyback circuit. By splitting the secondary windings ofthe flyback transformer used in the low voltage flyback circuit into twosections and by adding a switch circuit between the two sections, it ispossible to toggle the DC output current provided by the low voltageflyback circuit to the LED load between two different values.

In yet further embodiments, although the improved buck PFC circuit, thepassive voltage multiplier circuit, and the dual-output low voltageflyback circuit are designed and explained in conjunction with their useas components usable in an LED driver, it will be appreciated by thoseof skill in the art that each independent circuit design and teachinghas broad utility and can be used as components in a wide range of powersupplies, power converters, driver circuits, and for providing power toa wide variety of loads, other than just LED loads.

As an example of one of the above embodiments of the invention, thepassive voltage multiplier circuit is preferably configured a Zenerresistor multiplier arrangement that can be made to approximate anypolynomial function, not just a squaring function, such as X^(n) where“n” can be any real number. This arrangement can be added to existingdesigns of PFC converters (other than just buck PFC converters), such asflyback PFC converters, and can improve the THD results associated withsuch PFC converters, which reduces the current that must be drawn fromthe AC power supply. This multiplier circuit is typically configured tobe connected to the input voltage sensing pin of the control chip of thePFC converter.

Yet further, it will be appreciated by those of skill in the art thatthe improvements described herein can be used to advantage not only intwo-stage LED drivers or other types of power supplies but also insingle-stage LED driver and other power supply designs, as will beapparent based on the teachings contained herein.

Further features and advantages of the invention, as well as thestructure and operation of various embodiments of the invention, aredescribed in detail below with reference to the accompanying drawings.It is noted that the invention is not limited to the specificembodiments described herein. Such embodiments are presented herein forillustrative purposes only. Additional embodiments will be apparent topersons skilled in the relevant art(s) based on the teachings containedherein.

IV. BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the relevant art(s) to makeand use the invention.

FIG. 1 illustrates a conventional two-stage LED driver in block diagramformat;

FIG. 2 is a simplified schematic diagram of an improved LED driver inaccordance with embodiments of the present invention;

FIG. 3 illustrates two graphs, the top graph illustrating an idealcurrent provided by an AC power supply, the lower graph illustrating theactual output current drawn by a conventional buck topology PFC circuitwhen used with a low cost PFC controller with conventional sensed inputvoltage provided to the PFC controller;

FIG. 4 illustrates two graphs, the top graph illustrating an idealcurrent provided by an AC power supply, the lower graph illustrating theactual output current drawn by a conventional buck topology PFC circuitwhen used with a low cost PFC controller, but for which the sensed inputvoltage provided to the PFC controller is multiplied using a passivemultiplier circuit in accordance with embodiments of the presentinvention;

FIG. 5 illustrates an exemplary, passive voltage multiplier circuit usedwith the improved buck topology PFC circuit constructed in accordancewith embodiments of the present invention;

FIG. 6 is a schematic of an improved low-voltage flyback converterconstructed in accordance with embodiments of the present invention;

FIG. 7 illustrates an electrical schematic of a conventional, high powerflyback, single-stage LED driver;

FIG. 8 is a graph illustrating the current waveform generated by the LEDdriver of FIG. 7;

FIG. 9 is a graph of the line current generated by the LED driver ofFIG. 7 with varying values of Kv;

FIG. 10 illustrates an electrical schematic of an improved, high powerflyback, single-stage LED driver having an exemplary, passive voltagemultiplier circuit constructed in accordance with embodiments of thepresent invention; and

FIG. 11 illustrates an electrical schematic in block diagram format ofan improved, high power flyback, single-stage LED driver having both anexemplary, passive voltage multiplier circuit and an output currentsplitter switch circuit constructed in accordance with embodiments ofthe present invention.

V. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

While the present invention is described herein with illustrativeembodiments for particular applications, it should be understood thatthe invention is not limited thereto. Those skilled in the art withaccess to the teachings provided herein will recognize additionalmodifications, applications, and embodiments within the scope thereofand additional fields in which the invention would be of significantutility.

FIG. 1 illustrates a conventional two-stage LED driver 100 in blockdiagram format. The conventional LED driver 100 is connected between anAC power supply 25 and a load 95, which, in this application, is one ormore LEDs (hereinafter referred to either as “the load” or as “the LEDload”).

The AC voltage and current from the AC power supply 25 runs firstthrough a bridge or full-wave rectifier 30 and a high frequency inputfilter 35, which filters out the high frequency components generated bythe PFC circuit 140 and prevents such high frequency noise from beinginjected back into the bridge rectifier 30. Although shown as part ofthe LED driver 100, in some embodiments the bridge rectifier 30 andinput filter 35 are separate components through which the AC power issupplied before reaching the main components of the two-stage LED driver100.

The first stage of the conventional two-stage LED driver 100 is a PFCcircuit 140. The PFC circuit 140 is conventionally an active PFC, suchas a flyback converter in discontinuous conduction mode (DCM), a flybackconverter in transition or critical conduction mode (CrCM), a boostconverter in continuous conduction mode (CCM), a boost converter in DCM,or a boost converter in transition or CrCM.

The conventional PFC circuit 140 provides a near unity power factor (PF)and a low THD across the entire input voltage range from the AC powersupply 25. The second stage of the conventional two-stage LED driver 100is a DC-DC circuit 170, which may also be referred to as a downstreamisolation and regulator circuit, since it is configured to receive theDC voltage and the DC current output by the active PFC circuit and thento provide tight regulation or control over the output current (e.g., asubstantially constant current) provided by the LED driver 100 to theLED load 95.

In embodiments of the invention, FIG. 2 illustrates a simplifiedschematic of an improved LED driver 200 according to the teachingsdisclosed herein. The improved LED driver 200 is connected between theAC power supply 25 and the LED load 95.

As with the conventional LED driver 100 from FIG. 1, the AC voltage andcurrent from the AC power supply 25 runs first through the bridgerectifier 30 and the high frequency input filter 35. As with theconventional LED driver 100 from FIG. 1, in some embodiments the bridgerectifier 30 and input filter 35 may be part of the front end (or firststage) of the improved LED driver 200 or they may be separate componentsthrough which the AC power is supplied before reaching the maincomponents of the improved LED driver 200.

The improved LED driver 200 preferably includes a buck topology PFCcircuit 240. Preferably, the buck topology PFC circuit 240 is an activePFC; preferably configured as a buck converter operating in transitionmode or CrCM. A second, follow-up stage of the improved LED driver 200is a downstream isolation and regulator circuit, preferably in the formof a low voltage flyback converter 270 configured to provide a constantor substantially constant output current, at a desired amperage (such as350 mA or 700 mA, for example), to the LED load 95.

The buck topology PFC circuit 240 preferably includes a low-cost switch242, such as a 500 volt, 1.6 ohm MOSFET. As will be discussed in greaterdetail hereinafter, the buck topology PFC circuit 240 preferably alsoincludes a low cost current-mode PFC controller operating in transitionmode, such as an L6561 or L6562, either of which are available fromSTMicroelectronics, which is headquartered in Geneva, Switzerland andaccessible on the Internet at http://www.st.com.

Alternatively, instead of an L6561 or L6562 controller, the bucktopology PFC circuit 240 could also use a UCC28810 controller, which iseffectively comparable to either the L6561 or L6562 controller, andwhich is available from Texas Instruments Incorporated, headquartered inDallas, Tex., USA and accessible on the Internet at http://www.ti.com.

Although not listed herein, it will be appreciated by those of skill inthe art that there are many other, low-cost, current-mode PFCcontrollers, in addition to the L6561 or L6562 controller or theUCC28810 controller, that are comparable and could be used in place ofthe L6561 or L6562 controller or the UCC28810 controller describedherein.

The buck topology PFC circuit 240 provides superiorefficiency—maintaining a 96% or better efficiency at full-load (e.g., ata 95 W load) for AC line input voltages ranging between 90-270 VAC andwith the output voltage maintained at 80 VDC; and maintain a 95% orbetter efficiency with an AC line input voltage of 115 VAC at less-thanfull loads ranging between 30 W and 90 W and with the output voltagemaintained at 80 VDC.

A disadvantage of a buck topology PFC circuit 240, as compared to aboost converter, is that using one of the low cost controllers describedabove results in poor THD results. Poor THD in the AC input currentdrawn by the LED driver 200 causes the LED driver 200 to draw excessivepower and current from the AC power supply 25.

Although there are active PFC controllers, offered by several differentmanufacturers, that are specifically designed to improve the THD of theAC input current drawn by the buck topology PFC circuit 240, suchcontrollers are much more expensive (e.g., on the order of 3-4 timesmore expensive per controller) when compared to the low costcontrollers, described above, that are traditionally designed for usewith boost or flyback PFC circuits.

Since cost is a critical factor in the design of any LED driver, it wasdesirable to find a low cost solution to improve the THD of the input ACcurrent drawn by the buck topology PFC circuit 240 while still using oneof the low cost controllers that are not internally configured toimprove the THD of the input AC current when used with a buck PFCconverter.

According to their design specifications, the input peak current of abuck topology PFC circuit 240 is defined by the equation <Ig>=½*Ip*Vo/Vgwhere Ip is the programmed input current provided by the low costcontroller, such as the ST6562 chip, where Vo is the output voltage andVg is the input voltage to this stage. Further, the input peak currentfrom the low cost controller is defined by the equation Ip(t)=K*Vg(t).

Thus, when a low cost controller, such as the L6562 chip, is used withthe typical buck topology PFC circuit 240, the input peak current of thetypical buck topology PFC circuit 240 is defined as <Ig>=½K*Vo, whichgenerates a flat top current 300, as shown in the lower graph of FIG. 3.However, it was determined that if the sensed input voltage Vg wassquared (or otherwise multiplied by a factor of N, where N is a wholenumber equal to or greater than 2) as provided to the sensed inputvoltage pin of the low cost controller, such as the L6562 chip, then theinput peak current from the low cost controller could be re-defined bythe equation Ip(t)=K*Vg²(t). If that modification were made to thesensed voltage input into the low cost controller, then the averageinput current of the typical buck topology PFC circuit 240 can bere-defined as <Ig>=½K*Vg*Vo or a constant C*Vg, such that <Ig> followsVg, which indicates a unity (or near unity) power factor.

Merely squaring the sensed Vg input into the low cost controllerimproves the input AC current drawn by the improved buck topology PFCcircuit 240, which then draws a nearly sinusoidal input AC current 400,as shown in the lower graph of FIG. 4. Whereas the typical THD of the ACinput current drawn by a buck topology PFC circuit 240, using one of thelow cost controllers, would generally result in a 40-50% distortionrange (again, as shown by the lower graph of FIG. 3), it has been foundthat by squaring (or further multiplying) the sensed input voltageprovided to the low cost controller, even when used with the bucktopology PFC circuit 240, results in a THD of between 10% and 14% at aninput voltage ranging between 230 VAC and 115 VAC, where the outputvoltage for the 230 VAC input was maintained at approximately 80 VDC andwhere the output voltage for the 115 VAC input was maintained atapproximately 40 VDC.

FIG. 5 illustrates an exemplary, low cost, passive circuit 500 forsquaring the sensed input voltage Vg provided as an input to one of thelow cost controllers, such as the L6562 chip. Preferably, the exemplaryvoltage squarer circuit 500 is positioned between the full-waverectifier and the sensed voltage input (or input pin) of the PFCcontroller. The exemplary voltage squarer circuit 500 is preferablyimplemented as a Zener diode ladder.

The Zener diode ladder uses several lines to linearize thesquared-voltage function. For example, when the sensed voltage (Vin) islow, none of the Zener diodes conduct and the resistors determine thelowest slope of Vo/Vin. However, as Vin increases, the first Zener diodeconducts and imposes a constant current on its parallel resistor. Thus,the slope of Vo/Vin becomes steeper.

As Vin further increases, other Zener diodes begin to conduct currentand the Vo/Vin slope becomes steeper. In this manner, the squaredvoltage curve is approximated. Although not necessary, additional stepsof Zener diodes and parallel resistors may be added to the Zener diodeladder to improve the accuracy of the Vo/Vin slope. Advantageously, thispassive solution for squaring the sensed input voltage provided to thePFC controller is relatively cost effective since it does not requireany integrated circuits, which would increase the cost of the LED driver200. In other words, this passive voltage squaring solution enables thebuck topology PFC circuit 240 to be implemented with the lower costcontrollers, as described above, and without the need to use a moreexpensive controller (such as an IC) that has been designed specificallyto improve the THD of the AC current drawn by the buck topology PFCcircuit 240.

It will be appreciated by those of skill in the art that this passiveZener arrangement can be made to approximate any polynomial function(not just a squaring function), such as K*X^(n), where “n” can be anyreal number greater than 1 and K is a constant smaller than 1. Thisarrangement can be added to existing designs of other PFC converters,such as a flyback PFC converter, and can improve the input AC currentdrawn by such flyback PFC converters and the THD results for suchflyback PFC converters. Preferably, this voltage multiplier circuitarrangement is configured to receive the rectified input voltage fromthe full-wave rectifier and then to provide the modified (e.g., squared,cubed, etc.) rectified input voltage to the input voltage sensing pin ofthe PFC control chip, or PFC controller, used with the flyback PFCconverter.

Turning back to FIG. 2, the low-voltage, second stage flyback converter270 preferably includes a low-cost switch 272, such as a 150-200 volt,0.4 ohm MOSFET. The low voltage flyback converter 270 also includes aflyback transformer 280, in one embodiment being a 1:1 low leakage, highefficiency transformer. Additionally, the low voltage, second stageflyback converter 270 only needs to use a low cost, small capacitor 274,on the order of just 4.7 uF, by way of example. This is smaller, inorders of magnitude, compared to capacitors that are typically requiredby single-stage PFC designs (such as is required in the circuitsdescribed hereinafter and illustrated in FIGS. 7, 10 and 11).

Preferably, the output voltage from the buck topology PFC circuit 240 isdesigned to be between 40-80 VDC, which is the input DC voltage suppliedto the low voltage flyback converter 270. In one embodiment, the lowvoltage flyback converter 270 is, thus, designed to provide a constantor substantially constant current to the LED load 95 of 350 mA or 700mA, as desired based on the requirements and specifications of the LEDload 95 used in any particular application.

As will be appreciated by those of skill in the art, the exact outputcurrent can be configured, in advance, based on the winding ratios ofthe flyback transformer 280 and other design selections of otherconventional components within the low voltage flyback converter 270.

Advantageously, by squaring the sensed voltage input into the low costcontroller of the buck topology PFC circuit 240, the improved two-stageLED driver 200 is not only efficient, but draws an input AC current withgood THD across a wide range of input AC voltages. In addition, the sameor similar low cost controllers can be used for both stages 240, 270 ofthe two-stage LED driver 200.

Yet further, low cost MOSFET switches 242, 272 can be used with eachstage 240, 270, respectively, of the two-stage LED driver 200, and withthe two-stage design, there is no need for a high capacitance, moreexpensive load capacitor to minimize or filter the ripples of the outputcurrent provided to the LED load 95.

In another embodiment, modifications can be made to the low voltageflyback converter 270 to enable one LED driver to be able to switch ortoggle between two desired output currents provided to the LED load 95.

Turning now to FIG. 6, a schematic of a dual-current, low voltageflyback converter 600 is illustrated. This low voltage flyback converter600 may, but does not have to be, used in conjunction with the bucktopology PFC circuit 240, described above, as an alternative stage twocomponent of improved two-stage LED driver 200 from FIG. 2.

As shown in FIG. 6, the flyback transformer 680 is configured such thatits primary windings 610 are designed in conventional fashion; however,the secondary windings 620, 630 are preferably split into two identicalhalves (bifilar windings). A switch circuit 650, which includes a switchS1 and three fast Schottky diodes D1, D2, and D3, is placed across theoutput of the flyback transformer 680, such that the secondary windings620, 630 are configured either to be in series or in parallel with eachother—based on the position and status of the switch circuit 650.

More specifically, when the switch S1 is closed, the fast Schottkydiodes D1, D2 are in their OFF state, but the fast Schottky diode D3 isin its ON state, which causes the secondary windings 620, 630 to be in aseries configuration. Conversely, when the switch S1 is open, the fastSchottky diodes D1, D2 are in their ON state, and the fast Schottkydiode D3 is in its OFF state, which causes the secondary windings 620,630 to be in a parallel configuration.

Thus, when the secondary windings 620, 630 are in series, the outputvoltage from the low voltage of flyback converter 270 will be twice andthe amperage of the current will be “I” across the load 95. When thesecondary windings 620, 630 are in parallel, the output voltage of thelow voltage of flyback converter 270 will be half, but amperage of thecurrent across the load 95 will be doubled, in other words, two times“I” (i.e., 2×I, 2*I, or 2I).

Thus, by way of example only, if current “I” is set to 700 mA, suchcurrent is provided as an output of the low voltage of flyback converter270 to the LED load 95 when the switch circuit 650 is closed. However,when the switch circuit 650 is opened, the current provided as an outputof the low voltage of flyback converter 270 to the LED load 95 is equalto 2*I, or 1400 mA, in this example.

Typically, an LED driver is configured to deliver one specific outputcurrent. The above circuit design, however, enables a single LED driverto be switchable or to be set to one of two different output currents.Such a design is efficient, saves space, and improves logistics becauseone SKU LED driver may be used for two different output currentrequirements.

In addition, no modifications need to be made to the controller 660 toachieve the desired output current. As will be appreciated by those ofskill in the art, the specific output current “I” (and 2*I) can bedetermined, in advance, by the circuit designer based on the windingratios between the primary windings 610 and the secondary windings 620,630 of the flyback transformer 680 and other design selections of otherconventional components within the low voltage flyback converter 600.

In another embodiment, it is possible (i) to improve the THD or (ii) toprovide for a switchable output current or (iii) to combine bothimprovements in a single stage flyback converter or single stage flybackLED driver. A conventional single stage flyback LED driver 700 isillustrated in FIG. 7. The conventional single stage flyback LED driver700 has a high power factor and uses a conventional PFC controller 760,such as the L6561 (or an L6562 or a UCC28810). Most flyback PFC intransition mode use conventional controllers to sense the rectified linevoltage (Vin). Typically, this is accomplished by using a simpleresistor divider, such that Vsense=K*Vin where K<<1.

In this operation mode, the THD is not zero and the average inputcurrent does not precisely follow the input voltage. In other words, thecurrent signal is not sinusoidal, but has a flattened peak waveform. Theamount of this flatness is determined by Kv, meaning—the current signalis impacted by the input voltage magnitude with respect to the outputvoltage of the converter and the turns ratio of the main Flybacktransformer (n=N₂/N₁).

As shown in FIG. 8, the input current wave shape 800 can be partiallycured by “cheating” the input multiplier and feeding it with a counter810—flat signal, a pre-distorted sinusoidal input—that will force theflyback converter to draw more current at the peak, where it is normallyflat. For instance, a useful such distortion can be Vsense=K*Vin wheren>1 (where n=1 represents the conventional case with a simple voltagedivider). Still referring to FIG. 8, the primary current peak envelopeis represented by waveform 820 and the secondary current peak envelopeis represented by waveform 830. This results in an average primarycurrent 840, as shown.

The impact of the value of Kv on the line current is illustrated in thegraph 900 of FIG. 9. Specifically, different line current waveforms areillustrated based on the value of Kv. Specifically, different linecurrent waveforms are illustrated for the following values of Kv, whereKv=0.5, Kv=1, Kv=2, and Kv=4.

In one embodiment, an improved single stage flyback LED driver 1000, inwhich the THD of the converter is improved over a conventional singlestage flyback LED driver, is illustrated in FIG. 10. The improved singlestage flyback LED driver 1000 has a high power factor and uses aconventional PFC controller 1060, such as the L6561 (or an L6562 or aUCC28810). However, instead of just having a simple resistor leadinginto the sensed, rectified line voltage input or pin of the PFCcontroller 1060, the improved single stage flyback LED driver 1000includes a two Zener ladder 1500. Use of the Zener ladder 1500 improvesthe THD of the input AC current drawn by the single stage flybackconverter.

By way of example, with a Pout of 33 W and a Vin of 230 VAC, theconventional single-stage flyback LED driver 700 from FIG. 7 results ina THD of approximately 18%. In contrast, by using the Zener ladder 1500,with the same Pout of 33 W and the same Vin of 230 VAC, the improvedsingle-stage flyback LED driver 1000 from FIG. 10 results in an improvedTHD of approximately 7%.

As stated previously, it is possible (i) to improve the THD or (ii) toprovide for a switchable output current or (iii) to combine bothimprovements in a single stage flyback converter or single stage flybackLED driver. A single stage flyback LED driver 1100, with the combinationof an improved THD and with a switchable output current, is illustratedin FIG. 11. For simplicity and ease of understanding, non-relevantcomponents (which are shown in more detail in FIGS. 7 and 10) are notincluded in the schematic illustrated in FIG. 11.

The single stage flyback LED driver 1100 receives AC voltage and currentfrom the AC power supply 25, which runs first through the bridgerectifier 30 and usually through high frequency input filter (notshown). A Zener diode ladder 1500 or voltage multiplier circuitmultiplies the voltage sensed by the conventional PFC controller 1160,in a manner as previously described with reference to the flybackconverter from FIG. 10. Use of the voltage multiplier to modify theinput of the sensed voltage input or pin of the PFC controller 1160improves the THD of the flyback converter, as previously discussed.

The single stage flyback LED driver 1100 further includes a flybacktransformer 1180. As with the schematic of the dual-current, low voltageflyback converter 600 from FIG. 6, the flyback transformer 1180 isconfigured such that its primary windings are designed in conventionalfashion; however, the secondary windings are preferably split into twoidentical halves (bifilar windings). A switch circuit 1150, whichincludes a switch S1 and three fast Schottky diodes D1, D2, and D3, isplaced across the output of the flyback transformer 1180, such that thesecondary windings are configured either to be in series or in parallelwith each other—based on the position and status of the switch circuit1150.

More specifically, when the switch S1 is closed, the fast Schottkydiodes D1, D2 are in their OFF state, but the fast Schottky diode D3 isin its ON state, which causes the secondary windings to be in a seriesconfiguration. Conversely, when the switch S1 is open, the fast Schottkydiodes D1, D2 are in their ON state, and the fast Schottky diode D3 isin its OFF state, which causes the secondary windings to be in aparallel configuration.

Thus, when the secondary windings are in series, the output current ofthe single stage flyback LED driver 1100 will be set to a value of “I”to the load 95. When the secondary windings are in parallel, the outputcurrent of the single stage flyback LED driver 1100 will be doubled(2*I) to the load 95.

Thus, by way of example only, if current “I” is set to 700 mA, suchcurrent is provided to the LED load 95 when the switch circuit 1150 isclosed. However, when the switch circuit 1150 is opened, the currentprovided to the LED load 95 is equal to 2*I, or 1400 mA, in thisexample.

In addition, no modifications need to be made to the controller 1160 toachieve the desired output current. As will be appreciated by those ofskill in the art, the specific output current “I” (and 2*I) can bedetermined, in advance, by the circuit designer based on the windingratios between the primary windings and the secondary windings of theflyback transformer 1180 and by the control current (2*I) established bythe PFC controller 1160.

In contrast with output capacitor used with the dual stage flybackconverter 600 from FIG. 6, the single stage flyback LED driver 1100requires that the output capacitor (across the load 95) be larger thanis required when the PFC converter is designed as a two stage converter.For example, in the single stage flyback LED driver 1100 of FIG. 11, acapacitor having a capacitance between 1000 and 4700 uF would typicallybe necessary.

CONCLUSION

As noted above, embodiments of the present invention provide an improvedtwo-stage LED driver for powering an LED load. In some embodiments, animproved first stage of an LED driver includes a buck topology PFCcircuit that uses a low cost PFC controller that typically has a poorinput THD.

By squaring the rectified sensed input voltage provided to the PFCcontroller, preferably using a low cost, passive circuit, the THD of theAC input current drawn by the LED driver is significantly improved,while maintaining the efficiency of the buck PFC circuit, but alsowithout significantly increasing the overall cost of the LED driver.

In some embodiments, an improved second stage of an LED driver includesa low voltage flyback circuit. By splitting the secondary windings ofthe flyback transformer used in the low voltage flyback circuit into twosections and by adding a switch circuit between the two sections, it ispossible to toggle the DC output current provided to the LED loadbetween two different values.

The improved first and second stages of the LED driver can be used inconjunction with each other or can be used independently of each otherwithin an LED driver.

Further, although the improved buck PFC circuit, the passive voltagesquaring circuit, and the dual-output low voltage flyback circuit aredesigned and explained in conjunction with their use in an LED driver,it will be appreciated by those of skill in the art that eachindependent circuit design and teaching has broad utility and can beused in a wide range of power supplies, power converters, drivercircuits, and for providing power to a wide variety of loads, other thanjust an LED load.

The present invention has been described above with the aid offunctional building blocks illustrating the implementation of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

For example, various aspects of the present invention can be implementedby software, firmware, hardware (or hardware represented by softwaresuch, as for example, Verilog or hardware description languageinstructions), or a combination thereof. After reading this description,it will become apparent to a person skilled in the relevant art how toimplement the invention using other computer systems and/or computerarchitectures.

It is to be appreciated that the Detailed Description section, and notthe Summary and Abstract sections, is intended to be used to interpretthe claims. The Summary and Abstract sections may set forth one or more,but not all, exemplary embodiments of the present invention ascontemplated by the inventor(s), and thus, are not intended to limit thepresent invention and the appended claims in any way.

What is claimed is:
 1. A light emitting diode (LED) driver for poweringan LED load at a substantially constant current, comprising: a buckpower factor correction (PFC) circuit, having a PFC controller, the buckPFC circuit configured to (i) draw an alternating current (AC) inputcurrent having a first total harmonic distortion (THD), (ii) sense arectified input voltage from a full-wave rectifier at the PFCcontroller, and (iii) output a direct current (DC) voltage and a DCcurrent; a passive voltage multiplier circuit (i) disposed between thefull-wave rectifier and an input of the PFC controller, and (ii)configured to multiply the sensed rectified input voltage by a wholenumber greater than 1, the drawn AC current having a second THDdifferent from the first THD; and a downstream isolation and regulatorcircuit configured to (i) receive the DC voltage and the DC currentoutput by the buck PFC circuit, based on the drawn AC current, and (ii)selectively switch, by a switch thereof, the DC current betweendifferent values based on a status position of the switch, to providethe substantially constant current of a desired value to the LED load.2. The LED driver of claim 1, wherein the passive voltage multipliercircuit comprises a Zener diode ladder that includes a plurality ofZener diode-resistor pairs and wherein the Zener diode ladderapproximates a polynomial function for multiplying the rectified inputvoltage to be sensed by the PFC controller, the polynomial function isdefined as X″, where “n” is a real number corresponding to the number ofZener diode-resistor pairs in the Zener diode ladder.
 3. The LED driverof claim 2, wherein the input AC current having the second THD isdefined by the equation: Ip(t) =K*Vg^(n)(t), wherein K represents aconstant, Vg represents the input voltage and n is a whole number equalto or greater than
 2. 4. The LED driver of claim 1, wherein the passivevoltage multiplier circuit is part of the buck PFC circuit and whereinthe buck PFC circuit is configured to operate in transition mode.
 5. TheLED driver of claim 1, wherein the LED driver is a two-stage LED driver,wherein the buck PFC circuit is a first stage of the two-stage LEDdriver and wherein the downstream isolation and regulator circuit is asecond stage of the two-stage LED driver.
 6. The LED driver of claim 1,wherein the downstream isolation and regulator circuit includes a lowvoltage flyback circuit, wherein the amperage of the substantiallyconstant current of the desired value to be provided to the LED load isset according to a configuration of the low voltage flyback circuit. 7.The LED driver of claim 1, wherein the full wave rectifier is coupled toan AC power supply.
 8. The LED driver of claim 7, wherein the second,improved THD of the input AC current is lower than the first THD andwherein the second, improved THD enables the LED driver to draw lesscurrent from the AC power supply.
 9. A method for powering a lightemitting diode (LED) load at a substantially constant current using atwo stage LED driver, comprising: providing alternating current (AC)power from an AC power supply to a first stage of the LED driver, thefirst stage including a buck power factor correction (PFC) circuit and aPFC controller, the first stage of the LED driver drawing an input ACcurrent having a first total harmonic distortion (THD); generating arectified input voltage from the AC power supply; multiplying therectified input voltage by a whole number greater than 1, by a passivevoltage multiplier circuit disposed between a full-wave rectifier andinput to the PFC controller, the drawn input AC current having a secondTHD different from the first THD; sensing the multiplied rectified inputvoltage at the PFC controller; receiving a direct current (DC) voltageand a DC current output from the buck PFC circuit, based on the drawn ACcurrent, at a second stage of the LED driver; and selectively switching,by a switch of a downstream isolation and regulator circuit of thesecond stage, the DC current between different values based on a statusposition of the switch, to provide the substantially constant current ofa desired value to the LED load.
 10. The method for powering the LEDload of claim 9, wherein multiplying the rectified input voltage isperformed using a passive voltage multiplier circuit.
 11. The method forpowering the LED load of claim 9, wherein multiplying the rectifiedinput voltage is performed using a Zener diode ladder having a pluralityof Zener diode-resistor pairs, each respective Zener diode-resistor pairincluding a Zener diode in parallel with a corresponding resistor,wherein the Zener diode ladder approximates a polynomial function formultiplying the rectified input voltage to be sensed at the PFCcontroller, the polynomial function defined as X″, where “n” is a realnumber corresponding to the number of Zener diode-resistor pairs in theZener diode ladder.
 12. The method for powering the LED load of claim 9,wherein generating the rectified input voltage from the AC power supplyfurther comprises coupling the AC power supply to a bridge rectifier.13. The method for powering the LED load of claim 9, further comprisingoperating the buck PFC circuit in transition mode.
 14. The method forpowering the LED load of claim 9, wherein sensing the multipliedrectified input voltage at the PFC controller comprises providing themultiplied rectified input voltage to a voltage sensing input of the PFCcontroller.
 15. The method for powering the LED load of claim 9, whereinthe downstream isolation and regulator circuit includes a low voltageflyback circuit, further comprising setting the amperage of thesubstantially constant current to the desired value to be provided tothe LED load based on configuration settings of the low voltage flybackcircuit.
 16. A system for powering a light emitting diode (LED) load ata substantially constant current, comprising: an alternating current(AC) power supply that provides an input AC voltage and an input ACcurrent; a full-wave rectifier coupled to the AC power supply thatconverts the input AC voltage into a rectified input voltage; a firststage of a two stage LED driver, the first stage including a buck powerfactor correction (PFC) circuit operating in transition mode and a PFCcontroller, the buck PFC circuit configured to: (i) draw the input ACcurrent from the AC power supply at a first total harmonic distortion(THD), (ii) receive the rectified input voltage from the full-waverectifier at an input of the PFC controller, and (iii) output a directcurrent (DC) voltage and a DC current; a passive voltage multipliercircuit positioned between the full-wave rectifier and the input of thePFC controller of the first stage, and configured to multiply the sensedrectified input voltage by a whole number greater than 1, the drawninput AC current having a second THD different from the first THD; asecond stage of the two stage LED driver, the second stage including adownstream isolation and regulator circuit configured to: (i) receivethe DC voltage and the DC current output by the buck PFC circuit, basedon the drawn AC current, and (ii) selectively switch, by a switchthereof, the DC current between different values based on a statusposition of the switch, to provide the substantially constant current ofa desired value to the LED load.
 17. The system for powering the LEDload of claim 16, wherein the passive voltage multiplier circuitcomprises a Zener diode ladder.
 18. The system for powering the LED loadof claim 17, wherein the Zener diode ladder includes a plurality ofZener diode-resistor pairs, each respective Zener diode-resistor pairincluding a Zener diode in parallel with a corresponding resistor,wherein the Zener diode ladder approximates a polynomial function formultiplying the rectified input voltage to be sensed at the PFCcontroller, the polynomial function defined as X″, where “n” is a realnumber corresponding to the number of Zener diode-resistor pairs in theZener diode ladder.
 19. The system for powering the LED load of claim18, wherein the input AC current having the second, improved THD isdefined by the equation: Ip(t) =K*Vg^(n)(t) wherein K represents aconstant, Vg represents the input voltage and n represents a wholenumber equal to or greater than
 2. 20. The system for powering the LEDload of claim 16, wherein the downstream isolation and regulator circuitincludes a low voltage flyback circuit and wherein the amperage of thesubstantially constant current of the desired value to be provided tothe LED load is set according to a configuration of the low voltageflyback circuit.
 21. A light emitting diode (LED) driver for powering anLED load at a substantially constant current, comprising: a buck powerfactor correction (PFC) circuit, having a PFC controller, the buck PFCcircuit configured to: (i) draw an alternating current (AC) inputcurrent having a first THD, (ii) sense a rectified input voltage from afull-wave rectifier at the PFC controller, and (iii) output a directcurrent (DC) voltage and a DC current; a passive voltage multipliercircuit disposed between the full-wave rectifier and an input of the PFCcontroller, and configured to multiply the sensed rectified inputvoltage by a whole number greater than 1, the drawn input AC currenthaving a second THD different from the first THD; and a dual low voltageflyback circuit configured to: (i) receive the DC voltage and the DCcurrent output by the buck PFC circuit, based on the drawn AC current,and (ii) selectively switch, by a switch thereof, the DC current betweendifferent values based on a status position of the switch, and supplyingthe selected value to the LED load.